As has been well known in the art, a 13Cr steel containing approximately 0.2% of C and approximately 13% of Cr (this steel is referred to below simply as a “13Cr steel”) has excellent resistance to corrosion by infiltrating carbon dioxide gas, and it is less expensive compared to a super 13Cr steel having a decreased C content or a duplex stainless steel, so it is much used for oil country tubular goods (oil well tubing, casing, and drill pipes). A seamless steel pipe made of this 13Cr steel is manufactured by hot working a billet to form a mother pipe followed by quenching and annealing.
Patent Document 1 discloses a method for increasing toughness by the direct quenching technique (DQT) in which quenching is carried out immediately after a 13Cr steel has been hot worked to form a mother pipe. This method does not involve reheating of a mother pipe which has been cooled prior to performing quenching, and thus it is advantageous from the standpoints of productivity and costs.    Patent Document 1: JP H02-277720 A1